Zadie Smith’s fourth novel opens in Willesden, the territory of her celebrated debut White Teeth. Leah Hanwell, a local woman in her mid-thirties, is lying in a hammock, idling through the weekend papers. She hears a pleasing phrase on the radio — “I am the sole author of the dictionary that defines me” — and tries to write it in pencil on the back of a magazine. Only the pencil slips on the glossy paper, meaning she has to keep going over the words.

I have never found Smith so engaging as this. The style is fluid, supple, present tense. It lets all the words of London — written, spoken and imagined — flow into a stream of consciousness that is at once bracingly experimental and very much what the city sounds like. It is complex writing that is a pleasure to read.

It also opens up the central theme of this captivating book. Are we the authors of our own dictionaries? Can we choose the language that defines us? Or are we doomed to lose our identities in the Babel of London?

The narrative centres on four characters who grew up on the Caldwell estate in Willesden in the Eighties and later make their homes in the postcode (hence “NW”). Leah Hanwell is a redhead with a sexy black hairdresser husband and a benignly pointless council job, “as faithful in her allegiance to this two-mile square of the city as other people are to their families, or their countries”. In the first section, we inhabit her thoughts as she frets about having children and brushes up against a local drug addict. Later, we close in on her childhood best friend, Natalie Blake, now a successful lawyer. She began life as Keisha Blake, with a God-fearing Jamaican mother and an absent father. In 184 little episodes, we find out how she changed her name and earned her success — only to find herself unfulfilled.

In between we meet Felix Cooper, a former addict trying to go clean, and finally Nathan Bogle, the girls’ childhood lust object, now rather more menacing. These are drifting characters, uncertain of their position, yet it is their drift that lends the novel its form. “The book was written without a plan,” Smith has said. “Early on I decided to let myself be led by whatever appeared in front of me as I was writing it. The strange thing about it was that though it is the least planned of my novels … it’s the book that has ended up feeling most planned.”

One of the frustrations of Smith’s earlier novels was her tendency to exert too much control over her characters, to overburden her plots with kooky detail. Here the vitality is never for its own sake — instead, it flows from the language itself. At a couple of points, it erupts into little poems, shaped to illustrate an apple tree or a mouth full of bad teeth.

That’s not to say it all works — strangely, the scenes fall flat when we leave the NW postcode. It is not a novel that announces its grand themes and as such it will not earn the attention that those other great London novels have done. And yet it is Smith’s most satisfying novel, funny, sexy, weird, full of acute social comedy, like London. She’s up there with the best around. It’s ridiculous that this isn’t on the Booker long-list.